1. Field of the Invention
The invention relates to a process for preparing isocyanatoalkylsilanes.
2. Description of the Related Art
For a considerable time there has been great interest in an economic method of preparing isocyanatoorganosilanes in high yields and purities. The stated compounds are of high economic importance.
Isocyanatosilanes can be used, for example, as adhesion promoters between organic polymers and inorganic materials. In particular, however, isocyanatoorganosilanes are used in industry for terminating organic polyols, e.g., polyether polyols, polyurethanes, polyesters or poly(meth)acrylates. The resulting alkoxysilane-terminated prepolymers cure on contact with atmospheric moisture and are used, among other things, as adhesives and sealants or else as film-forming resins or constituents thereof.
In the prior art in these contexts it is mostly conventional γ-isocyanatopropylsilanes of the general formula (1) that are usedOCN—(CH2)3—SiR′y(OR″)3-y  (1),where R′ and R″ are alkyl radicals and y is a number of 0-3, preferably 3 or 2.
More recently, however, particular interest has focused on the so-called α-isocyanatomethylsilanes of the general formula (2)OCN—(CH2)—SiR′y(OR″)3-y  (2),where R′ and R″ and y have the definitions stated above.
These α-isocyanatomethylsilanes feature particularly high reactivity to atmospheric moisture and are suitable for preparing alkoxysilane-terminated prepolymers with a high but regulable curing rate (described, for example, in EP 1 421 129). Moreover, the corresponding α-silane-terminated prepolymers can be crosslinked even without the tin catalysts which are controversial from the standpoint of toxicology (described inter alia in EP 1 421 129).
There are various known processes for preparing isocyanatoorganosilanes. EP 1 010 704, for instance, describes a process for preparing γ-isocyanatoorganosilanes that cleaves carbamatoorganosilanes in a combined cleaving and rectifying column at pressures of preferably 40-80 mbar to form the corresponding isocyanatoorganosilanes. That process adds tin-II chloride catalyst to the liquid phase. A disadvantage of that process is its very low conversion rates, which necessitate, moreover, a high level of cost and apparatus complexity in order to isolate and purify the reaction products. To date, therefore, this process has not been employed industrially.
DE 101 08 543 describes the preparation of isocyanatoorganosilanes from the corresponding carbamatoorganosilanes and alkyl- or vinylchlorosilanes. This process as well has proved unsuitable for industrial production and to date has therefore not been employed.
Known from EP 0 649 850 is a process in which carbamatoorganosilanes are cleaved thermally to form isocyanatoorganosilanes and methanol. This cleavage takes place in the gas phase under atmospheric or reduced pressure. The reaction is preferably carried out in a tube reactor, in which the evaporated carbamatosilane is heated to temperatures which lie well above the evaporation temperature of this silane. An improvement to this process, in which the carbamate cleavage is carried out in the presence of a heterogeneous catalyst, is known, furthermore, from EP 1 343 793. Disadvantages of these processes include the high thermal load on the highly reactive reaction product, and the high level of equipment and energy expenditure and complexity.
The thermal cleavage of carbamatoorganosilanes to form isocyanatoorganosilanes and methanol is described, moreover, in U.S. Pat. No. 6,008,396. There, carbamatoorganosilanes in inert hot media, with elimination of alcohol, are converted into the corresponding isocyanatosilanes, which are then removed directly from the reaction mixture by distillation. With this process as well, the reaction products are exposed to high thermal loads. Moreover, byproducts and impurities may accumulate in the inert medium.
A further process, in which the isocyanatoorganosilanes are prepared under the action of microwaves, is described in WO 2005/056564. In WO 2005/055974, finally, this microwave process is described in combination with fluidizing particulate solids. This process as well entails a high level of technical complexity and expense.